It’s worth checking if you have any 24/7 vets in the general area if you’re worried and want to contact someone sooner rather than later (these places usually take emergencies but they’re usually willing to answer questions over the phone as well in my experience), but if there isn’t one then the kittens should be fine for the night if they seem healthy still aside from the fleas. The main thing you have to worry about with fleas is anemia iirc, which can be deadly for kittens if left untreated. If they aren’t abnormally lethargic and don’t have pale gums then they’re likely not in immediate danger, just make sure to stay on top of it and get them treated (using professional advice) as soon as you’re able. Good luck!